| Fossil energy crisis is a major problem faced by all countries in the new century,so it is an urgent target to find new alternative energy sources.Marine energy,as a renewable energy source with a huge amount of energy,has naturally become the focus of people’s attention.The full exploitation and utilization of marine energy is an effective way to solve the current energy crisis.It is of great significance to research and develop devices and technologies for extracting marine energy.Marine current turbine has become today’s research hot,but most of the present study is based on the theory of wind turbine design model to develop water turbine,based on the practical underwater turbine working environment is not perfect,might be possible for the blade of dirt deposition and cavitation efficiency and service life of the decline is caused by the lack of special design research methods.Therefore,this paper focuses on the two main factors of roughness and cavitation to study the design theory and method of the special airfoil and blade for tidal energy turbines.Through numerical simulation methods and prototype model tests,the proposed design theory and the designed airfoil.The research on the hydrodynamic performance of the blade includes the following aspects:（1）Firstly,research on geometric curve modeling of airfoils has been carried out.For parametric modeling of airfoils,a non-uniform rational spline curve method was proposed to characterize airfoils,and carry out study on linear fitting to overcome the defects of the existing common methods,such as Bezier method,which had poor local control.The airfoil geometry line is interpolated and approximated respectively,and the airfoil curve rational approximation algorithm model is established.At the same time,based on the least square method and the control curve extreme value change rate,the model line fitting error and smoothness evaluation model are given.A mathematical model with the fitting error and curve smoothness as the objective function is established.Based on the above model,the penalty function and quasi-Newton algorithm have carried out airfoil curve optimization research.By analyzing the error and curvature evaluation of the fitted profile,it can be concluded that the fitting accuracy reaches grade 10-4 and meets the requirements of aerodynamic（hydrodynamic）mechanical experiment accuracy of airfoil.The smoothness of airfoil curve is also better than the result of curve interpolation,which provides a geometric numerical model for the subsequent research on the performance of special airfoil.（2）Secondly,carry out a special airfoil design methods and numerical simulation experiments for the roughness and cavitation factors faced by hydroturbines,establish comprehensive evaluation indexes for airfoil roughness sensitivity and cavitation performance.Perform a airfoil parameterization expression based on NURBS curve and combine design variables and constraint conditions,a multi-objective optimization mathematical model with the minimum pressure coefficient peak value and lift coefficient as the target under smooth and rough conditions is established,and the improved evolutionary algorithm strategy is used to perform global optimization.Using XFOIL to calculate and analyze the lift-drag coefficient and pressure coefficient of the designed airfoil,the results show that the designed airfoil has low roughness sensitivity and better anti-cavitation performance compared to the original airfoil.The results show that the designed airfoil has low roughness sensitivity and better anti-cavitation performance compared to the original airfoil.Based on the CFD method,the improved transition prediction model and the smoke flow test method,the flow field distribution of the airfoil at different angles of attack and Reynolds number and the effect of cavitation on the airfoil lift and drag coefficient are analyzed.The development and change of laminar separation bubbles and their influence on the aerodynamic performance of the airfoil compare and analyze the difference between the flow separation morphology of the designed airfoil and the traditional airfoil in the rough state.At last,hydrodynamics experiment is carried out on the designed airfoil through the water tunnel test platform,and the lift resistance coefficient is measured and compared with the numerical results.The results show that the agreement between the two results proves that the lift resistance performance of the designed airfoil is improved.（3）Thirdly,in view of the shortcomings of traditional blade element momentum theory（BEM）in the design of turbine blades,an improved blade element momentum theory（BEM）is proposed,and add a new blade tip loss factor correction model.Considering that the blade is affected by cavitation factors,in order to avoid or reduce the probability and intensity of cavitation on the surface of the blade,the established cavitation prediction model is combined with the improved BEM to form a new type of hydroturbine theoretical method of blade anti-cavitation design.At the same time,based on the design theory method,an optimized mathematical model with the maximum power coefficient and the minimum blade surface area as the establishment of objective function and the design of a 10k W hydroturbine prototype.The hydrodynamic performance of the hydroturbine is studied by CFD method,and the performance parameters such as the power coefficient and output torque of the designed blade and the reference blade established based on traditional theory under different tip speed ratios are compared and analyzed.The numerical results show that the turbine based on modified blade momentum theory and modern optimization method can improve the power coefficient,reduce the blade surface area and improve the anti-cavitation performance.In addition,the influence mechanism of the flow field distribution on the blade surface,the comparison of wake vortex shape and cavitation shape,and the hydrodynamic performance under unsteady conditions are studied,and the development and change law of the two wake fields are analyzed and compared.The feasibility and effectiveness of the design method are verified by the numerical simulation analysis of the performance of both.（4）Finally,based on the similarity criterion,design and manufacture the experimental model of the hydroturbine.Carry out visualization research of blade cavitation and comparative research of power generation performance test separately through the self-built cavitation water tunnel and flume test system.Through the visualization experiment to study the blade tip vortex cavitation under different working conditions and the rule of blade cloud cavitation formation and development,the difference of cavitation morphology and strength between the design blade and the reference blade can be analyzed and compared intuitively.The experimental results show that the cavitation area of the designed blade decreases compared with that of the reference blade under experimental conditions,which corresponds to the numerical results.To verified the performance difference between the design blade and the reference blade by measuring the power generation and torque of the blade under different tip speed ratios.The experimental results show that the output power of the design blade is higher than that of the reference blade under the design condition,and the performance difference between the two is verified by numerical simulation results,which shows the effectiveness of the proposed design method. |
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